Medical diagnostic instruments such as ophthalmoscopes, otoscopes, colposcopes and the like, are widely used in the health care field to examine both external regions and externally accessible internal regions of a patient, such as a human or an animal. The instrument typically includes an illuminating system for producing illuminating light, an imaging system for directing the illuminating light on a target, such as the retina of the eye of a patient, and for transmitting light reflected from the target to a viewing region, such as an eyepiece of the instrument viewed by a health care professional. The reflected light image from the target can also be transmitted to an electronic imager, such as a CCD or CMOS imager, which produces an electronic image that can be viewed on a display, that can be further processed, or that can be archived for future reference. (The following are of interest in disclosing eye diagnostic instruments which use electronic imagers: U.S. Pat. No. 4,146,310, issued Mar. 27, 1979, inventors Kohayakawa et al.; U.S. Pat. No. 6,334,682 B1, issued Jan. 1, 2002, inventor Takai; U.S. Pat. No. 6,309,068 B1, issued Oct. 30, 2001, inventor Kohayakawa; U.S. Pat. No. 6,685,317 B2, issued Feb. 3, 2004; and U.S. Patent Application Publication No. 2005/0110949 A1, published May 26, 2005, inventors Goldfain et al.).
Whether viewed live by a health care professional or captured by an electronic imager, the reflected light image of the target must be in focus for the viewer or for the electronic imager, in order to be usable. In known apparatus, focus of a reflected image is obtained by using an optical assembly that has one or more optical elements which can be moved relative to each other. This is desirable in an eyepiece of an ophthalmoscope, for example, in order to adjust the position of the eyepiece focal plane relative to the image of the retina of a patient's eye being viewed to compensate for the refractive error of either the patient or the health care professional. U.S. Pat. No. 6,390,625 B1, issued May 21, 2002, inventors Slawson et al., discloses a manually operable focusing mechanism for an optical instrument. Although suitable for the purposes for which it was intended, it would be desirable to provide a focusing mechanism that is simple, compact, provides the desired dynamic range and which has no moving parts.
A recent development in optics is the variable focus liquid lens disclosed in U.S. Pat. No. 6,369,954 B1, issued Apr. 9, 2002, inventors Berge et al. As disclosed, the variable focus liquid lens comprises a chamber filled with a first liquid, a drop of a second liquid being disposed at least on a region of a first surface of an insulating wall of the chamber, the first and second liquids being immiscible, of different optical indexes, and of substantially same density. The first liquid is conductive and the second liquid is insulating. The lens further comprises means for applying a voltage between the conductive first liquid and an electrode placed on the second surface of the wall; and centering means for maintaining the centering of the edge of the drop while the voltage is applied and for controlling the shape thereof. (See also: U.S. Patent Application Publication No. U.S. 20005/0002113 A1, published Jan. 6, 2005, inventor Berge.) U.S. Pat. No. 6,806,988, issued Oct. 19, 2004, inventors Onuki et al., discloses use of such a variable focus lens in a CCD imager system. The Varioptic Company, Lyon, France, a supplier of variable focus liquid lenses, suggests several medical equipment applications of such a lens on its website. Suggested uses are endoscopy, imaging by confocal microscopy, laser beam focus, control on tumours, and ophthalmology. (See also: the article “Variable focal lens controlled by an external voltage: An application of electrowetting”, by B. Berge and J. Pesaux, Eur. Phys. J. E. 3, 159-163 (2000), which suggests (page 163) “[E]ndoscopy could benefit from the compactness of the lens [i.e., variable focus liquid lens] - - - ”). However, none of these references disclose specific configurations as to how an electrically controllable variable focus liquid lens can be incorporated into an optical medical diagnostic instrument.
There is thus a need in medical diagnostic instruments of an optical focusing system that is simple, compact, provides the desired dynamic range, has no moving parts, consumes a minimum of electrical power, and can be readily incorporated into existing instrument designs.